I downloaded Python 3.1.3rc1 this morning to do my civic duty of testing it. I don't know what I'm doing wrong, but for me, test_io hangs and never completed. I'm on OS X 10.6.5. I ran it with:

$ ./python.exe Lib/test/regrtest.py test_io

And I got:

test_io

Testing large file ops skipped on darwin.
It requires 2147483648 bytes and a long time.
Use 'regrtest.py -u largefile test_io' to run it.

Testing large file ops skipped on darwin.
It requires 2147483648 bytes and a long time.
Use 'regrtest.py -u largefile test_io' to run it.
python.exe(12242) malloc: *** mmap(size=9223372036854775808) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
python.exe(12242) malloc: *** mmap(size=9223372036854775808) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
python.exe(12242) malloc: *** mmap(size=9223372036854775808) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
<hangs at this point>

I build Python with this configuration:

$ ./configure --enable-universalsdk=/ --enable-framework --with-universal-archs=intel

Oh, and it's being run as 64-bit.
By looking at the size being allocated, by guess that it's some kind of overflow thing or something.